A present day workstation configuration employs a cluster interconnection scheme in which multiple "slave" workstations utilize the database existing in a "master" unit. The cluster interface is implemented as a "daisy-chained" wired system, utilizing two digital signals, data and clock. A bidirectional electrical interface is used and a single jacketed cable containing two individually shielded twisted pairs is run from one workstation to the next, with the cables resistively terminated at the extreme ends of the line.
Communications along the line use a "poll and select" protocol, wherein the "master" periodically interrogates each "slave" to see if communication is desired. The "master" can direct data to a particular slave via its unit address. A bit-oriented protocol is used by all of the workstations. Transmissions are necessarily half-duplex along the clock and data busses. Transmission by any unit involves the enabling of its data and clock drivers and the generation of a predetermined signal pattern including a "flag" character. Receipt of the latter by the receiving stations is a prerequisite for the acceptance of transmittal information.
Each workstation in the cluster has an inherent communication bit rate. It is the function of the "master" to determine the maximum rate for the entire cluster, based upon the slowest "slave".
The wired cluster has several fundamental limitations. An overall cluster length limit, as well as a maximum distance between workstations must be imposed. This condition results from wire capacitance and the drive capability of the cluster interface circuits. Additionally, ground potential differences between units connected to different power sources introduce noise into the system which can, at times, cause so many data errors that cluster data cannot be transmitted at a high rate. Finally, wired cluster cables carrying sensitive data which pass through unprotected areas are subject to compromise.
What is required is an interface which is not subject to the foregoing limitations, performs all of the functions of the wired interface and is a direct replacement therefor. The fiber optic interface module of the present invention fills such a need.